1. SuperGen Assembly Cranfield University. 23rd Nov. 2016
SuperGen Assembly Cranfield University. . 23rd Nov Incident and Wake Turbulence and Interactions with Horizontal Axis Rotors.
Mike Graham,
Department of Aeronautics, Imperial College London
Acknowledgements: Rafael Palacios-Nieto, David Hankin (BMT FM). .

2. Summary:
HAWTs typically 3 bladed, upwind rotor machines, diameter up to ~150m, tip speed ~ 80+ms-1.
[HATT prototypes typically 2 bladed, up- & down-stream
rotors, diam. ~ 30m, tip speed O(10ms-1).]
Rotor blades are subject to Random loads due to incident Turbulence in the ABL or in wakes of other upstream turbines when deployed in farm arrays.
As part of SuperGen (MAXFARM) Imperial College is investigating aerodynamic control to reduce unsteady blade loading.
This presentation will deal with some issues of the turbulence impacting the rotor blades.

3. Rotor Wakes. Wind-Farm (Horns Rev, offshore) Denmark)

4. Flow Field Computation
URANS computation of the flow around a (downwind) HAWT. [Acknowledgement F. Zahle (Risoe Lab.)]
Iso-contours of absolute vorticity shown.
Tip and root vortices preserved well downstream of turbine when incident flow is uniform.
shedding frequency.

5. Aeroelastic Rotor Flow Simulation [Free Wake Panel Method – unsteady vortex lattice method (UVLM), FEA Beam element dynamics, Twin-bladed rotor].

6. Mesh for convected turbulence field

7. INCIDENT TURBULENCE (ABL)
Simulated Streamwise Velocity Component assuming von-Karman spectrum and Gaussian probability.
u velocity component
Von Karman spectrum

8. Power and Thrust Coefficient Response

9. INCIDENT TURBULENCE DISTORTION Schematic of Actuator Disc Mean Flow

11. Spectrum Suu of distorted small length-scale turbulence, ---- undistorted (von Karman spectrum).

13. Distortion of Turbulence into a HAWT Rotor (axis)
Spectrum of streamwise turbulent velocity Suu, Lx/Radius marked on curves, ---- undistorted.

14. Incident Wakes from Upwind Rotors
Mean flow and turbulence

15. Distortion of mean flow profile of rotor wake.
Mean ΔU on axis at rotor / ΔU∞
Velocity defect ΔU∞ = 0.25U∞

16. Turbulence and Wake Interactions (Wind Tunnel Expt.)

17. Wake of a three bladed rotor in uniform incident flow (UVLM free vortex wake)

18. Mean wake velocity profile U(y), versus experiment of Hancock & Pascheke (o)

19. Actuator disc or line representation of rotors for RANS computations of turbulence in wind farms.
Sinks of mean flow momentum.
Sources of turbulence KE. (κ)
Length-scale of rotor wake turbulence (ε)
Alternatively LES simulations (eg: Porte-Agel et al.)

21. Spectrum of turbulence in rotor wake 1D downstream of rotor.

22. Autocorrelation coefficient versus time separation in wake 1D downstream of rotor.

25. Turbulence Energy, Length scale and dissipation in rotor wake

26. Conclusions Distortion of turbulence only significant if length scale of turbulence < rotor diameter. Some distortion (< 15%) of mean wake profiles may be expected. Some empirical correlations for turbulence KE. (κ) and length-scale (dissipation ε) have been estimated for a rotor wake.